A typical positive photoresist is a generally two-component material consisting essentially of an alkaline soluble polymer and a radiation-sensitive dissolution inhibitor such as diazonaphthoquinone sulfonate (DNS), wherein the alkaline soluble polymer is rendered insoluble in aqueous alkaline solutions through addition of the radiation-sensitive dissolution inhibitor.
The most widely used positive photoresist is a novolac resin (a condensation polymer of formaldehyde and phenol) containing DNS. To date, several advanced IC manufactures require images that are stable up to 200.degree. C. or even higher. The novolac-based photoresists are unable to perform very well in this temperature range. Thus, some other phenolic polymers have been developed to serve as replacements for the conventional novolace-based photoresists. Example of these phenolic polymers include poly(p-hydroxystyrene) disclosed in U.S. Pat. Nos. 4,139,384 and 4,439,516; poly(p-hydroxy-.alpha.-methylstyrene) disclosed by H. Ito, et al, Macromolecules, 16, 513 (1983); and copolymers of N-(4-hydroxyphenyl)maleimide and various olefins disclosed by S. R. Turner, et al, Polym. Eng. Sci., 26, 1096 (1986). In general the alkaline soluble polymers contain carboxylic acids or maleimides as well as phenolic units. For example, poly(p-benzoic acid), disclosed by H. Ito, et al in "Advances in Resist Technology and Processing IV", SPIE, 771, 24 (1987), and maleimide/styrene copolymer, disclosed by C. E. Osuch, et al in "Advances in Resist Technology and Processing III", SPIE, 531, 68 (1986), have been employed to provide positive imaging of high temperature polymers. In addition, a maleimide/allyltrimethylsilane copolymer, disclosed by R. Sezi, et al, Polym. Eng, Sci., 29, 891 (1989), was used as the top imaging layer in a bilayer resist process because of its excellent durability during oxygen-plasma etching.
G. N. Taylor, et al (Solid State Technol., 27, 145 (1984) reported that small amounts of silicon (about 10 wt %) can drastically lower the oxygen-plasma etching rate of organic polymers.
We, in an article entitled "Preparation and Properties of Si-Containing Copolymer for Near-UV Resist. I. Poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilylstyrene 3", J. Polym. Sci., part A., 29, 399 (1991), and in a pending U.S. application Ser. No. 07/781,616, filed Oct. 23, 1991, disclosed a method to synthesize poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilylstyrene) (PHTMSS). The co-inventors of the present invention also, in an article entitled "Preparation and properties of silicon-Containing copolymers for Near-UV Resist II", Die Angewandte Makromolekulare Chemie, 205, 75-90 (1993), disclosed a method to synthesize poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilyl-.alpha.-methylstyre ne) (.alpha.-PHTMMS), and apply them as the top imaging layers. In the .alpha.-PHTMMS copolymer so disclosed, the N-(4-hydroxyphenyl)maleimide, which acted as a highly thermally stable unit, and the p-silylstyrene, which acted as an oxygen-plasma etching resistent unit, were alternatingly copolymerized. However, in both disclosures the silicon content of the PHTMSS and a .alpha.-PHTMMS copolymer is about 7 wt %.
As a result of an extensive investigation to develop a photoresist which is more thermally stable and more resistant to oxygen-plasma etching than prior art photoresists, a series of novel maleimide/silylstyrene copolymers have been newly synthesized in the present invention.
Accordingly, one object of the present invention is to provide new and useful maleimide/silylstyrene copolymers.
Another object of the present invention is to provide photoresist compositions containing the new maleimide/silylstyrene copolymers.
A further object of the present invention is to provide a process for preparing the new maleimide/silylstyrene copolymers.